An MRI apparatus for use in MRI is an apparatus which measures an NMR signal (echo signal) to be generated by nuclear spins constituting an object, in particular, a tissue of a human body, and images the form or function of the head, the abdomen, four limbs, or the like in a two-dimensional or three-dimensional manner. The echo signal is given different phase encode and frequency encode as positional information depending on a gradient magnetic field, and is arranged in a k space according to the positional information. The echo signal arranged in the k space is subjected to two-dimensional or three-dimensional Fourier transform, thereby reconstructing an image.
In MRI, the echo signal is measured so as to acquire data along a predetermined scan track of the k space. The scan track of the k space is classified roughly into a scan track by orthogonal system measurement which is determined by a gradient magnetic field pattern to be applied and acquires data on a k space of an orthogonal coordinate system, and a scan track by non-orthogonal system measurement which acquires data on a k space of a non-orthogonal coordinate system.
The k space of the orthogonal coordinate system is a two-dimensional or three-dimensional data space which is defined by an orthogonal two or three coordinate axes, and the k space of the non-orthogonal coordinate system is a two-dimensional or three-dimensional data space which is defined by size and declination. In the non-orthogonal system measurement, since the k space is scanned while changing the declination, near the center of the k space is repetitively scanned (for example, see NPL 1). Accordingly, this method is a robust measurement method in which the effect due to motion, such as breathing, is averaged, and no artifact is focused in a specific direction.
As an imaging method of MRI, an FSE method is known in which, after the application of a single excitation pulse, a plurality of reconvergence pulses are applied for TR until the application of the next excitation pulse to acquire a plurality of echo signals at high speed. In the FSE, the application of the single excitation pulse is referred to as a shot, and a plurality of echo signals obtained in one shot are referred to as an echo train. A method (hybrid radial method) which combines the non-orthogonal system measurement with the FSE method and obtains an image with few artifacts at high speed is known.
In the hybrid radial method, each echo train is subjected to orthogonal system measurement inside the k space of the rectangular orthogonal coordinate system referred to as a single blade, and a blade is rotated inside the k space for each shot. In this case, the major axis direction of the blade corresponds to frequency encode, and the minor axis direction of the blade corresponds to phase encode.
As a measurement method which fills the k space at high speed, an EPI method is known in which measurement is made by combining a read gradient magnetic field in a frequency encode direction and a blip gradient magnetic field in a phase encode direction. The non-orthogonal system measurement may also be combined with the EPI method. In this case, the minor axis direction of the blade is referred to as frequency encode, and the major axis of the blade is referred to as phase encode (for example, see PTL 1). By the combination of both methods, it is possible to suppress artifacts, to reduce each application time of the frequency encode gradient magnetic field, and to reduce image strain.